This question came up again in the context of a long discussion about case airflow techniiques, strangely entitled, question on an oddity ive noticed here. I decided to do some empirical testing to answer the question with 2 Nexus fans versus one. The following, slightly edited from the orignal, was posted deep in the above thread where it will never be found.

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NOTE: A basic rule of acoustic addition states that 2 identical sound sources will result in a 3 dB increase in overall noise. Four identical sound sources will result in +6 dB.

2 Nexus 120 fans, powered by a fanless PSU, in "the quiet room", in which the B&K SLM was seeing an ambient reading of 15~16 dBA (at dawn). Very hard to stop the meter from moving -- even a car 2 blocks away can have an impact when it's this quiet.

OK, so that shatters the idea that 2 identical noise sources means a 3 dB increase. Obviously not a universal rule! I learned something new here!

This also confirms my impression that the lab PC, which has a very slow Nexus 120 for exhaust and a Seasonic Super Tornado with Yate Loon 120 fan, is ~18 dBA/1m. I should really measure it.... so I did.... and got 18~19 dBA.

Time to try the Nexus fans at 12V.

from 1 meter:1 fan at 12V: 21 dBA.
2 fans at 12V: 22 dBA.

This is actually kind of amazing. It is not what I expected. So maybe the +3dBA rule only holds for louder noises? Or maybe it's the low freq balance and absence of midband of the noise that makes this rule not apply? Perhaps the rule is only valid without A weighting.

What did I hear at the 12V drive level? At 1m, the difference was very marginal, but I could hear it. From less than ~1/2m, the increased noise was more obvious but not big.

I tried the meter w/o any weighting. Levels jumped into the mid-30s and higher, and the needle was jumping up/down ~10 dB! I stood there and listend for a while to see if I could hear what the meter was picking up. 2 blocks away is Main St, by now starting to get busy. There was some distant rumbing that was audble but at a very low level. I am guessing that the meter was picking up on the low freq traffic rumbling <30Hz that was not so audible to me. In any case it was not usable w/o the low filter of the A weighting.

Time to try something more drastic: A much louder 120mm fan, which shall remain nameless for now.

from 1 meter:1 fan at 12V: 30 dBA.
2 fans at 12V: 33 dBA.

Yup, +3dB "rule" confirmed at this level -- with this fan. Tried the test both ways -- either of the fans read 30 dBA by itself. Adding the other always brought the reading up to 33 dBA. And there was no question of the audibility at 1 meter. Totally obvious. This fan also had a whole lot more noise in the midband.

My preliminary conclusions:

1) The +3dB rule is valid but only when not weighted by any compensation curve. (Like the "A" weighting we use.)
2) The +3dB rule does NOT always reflect human hearing perception.
3) The exceptions are when the noise is mostly in the low freq, or probably when it is very low in level, say under 20 dB in the midband.

Secondary conclusions:

The issue of multiple vs single fans is clear with the Nexus 120. Two of them at 6V move as much air as a single one at 12V, and make 2-3 dBA LESS NOISE!!

This jibes with my experience of Panaflo 80Ls... which I found puzzling for a while. My wife's PC has 3 80mm Panaflos -- one in the PSU, one on the HS and another at the front to keep the suspended B-IV drive cool. The fans all run at 5V or less. It measures 17~18 dBA/1m and sounds quieter than any PC I've built -- except for experiments with notebooks drives. I remember taking out the front fan; heard no difference so stuck it back in to keep the drive cool.

Again, I think it's a question of freq balance. When you slow any decent fan down enough, the sound shifts to a lower freq balance. This is true of every size of fan, I can vouch from years of experimentation. Both the whistling caused by blade edges and midband whooshing at high rpm drops dramatically at low speed. So the whole sound moves down in freq to where our hearing is less sensitive.

I think it is safe to say that when you combine low speed fans, that +3dB rule gets moot because...

1) your hearing is not sensitive to that noise because it's mostly low freqs. Certainly you don't hear volume changes as linearly as in the midband. So you just can't hear the increased acoustic energy / SPL even though it's there.
2) the audibility may be borderline because it's at or near ambient level, so again, you can't hear it or not well.

BTW, AFAIK, low freq sensitivity in human hearing does not change much with age -- except maybe for those who really elderly or with abnormal hearing loss/damage.

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In the original thread, NeilBlanchard followed up with this post:

Here is a link to an explaination of the Fletcher Munson curves. Huh? What does this have to do with this discussion?

This phenomenon of rapidly decreased ability to hear low frequency sounds as loudness level drops is precisely what is reflected in my experiments here. The shrewd PC silencer will want to exploit this aspect of human hearing to maximize cooling airflow while still keeping audibility to a minimum. -------------------------------

After which I added:

I see the +3dB rule as an interesting physical phenomenon that can have a bearing on what we hear. By itself, +3dB rule does not dictate what we hear; it is simply a relationship between physical phenomenon and the meaurement standards/tools we have. The acoustic energy in the space is much less important than what you hear -- the 2 are not the same at all, far from it.

The A weighed dB scale does not try to read the acoustic energy (SPL is not reallly an energy measurement anyway...); rather, A weighting is an attempt to simulate the human hearing frequency response documented by Fletcher Munson (thank you NeilBlanchard )and others. It's a bit rough, but much better than unweighted SPL readings, which have almost no correlation to what we hear when levels / freqs are low.

So when I measured 15~16 dBA ambient, then got 16~17 dBA with one or both fans turned on a meter away, this is very very close to what I heard. IE, I could not tell whether one fan was on or two fans from a meter away. I had to get within ~a foot to hear that difference clearly. The dBA readings did a very good job of simulating what I heard, and this is what really matters: the measurements jibe with what I hear. If the measurement don't reflect with what I hear, then, for our purposes, the measuring tool/system has to be made better -- NOT the other way around.

The issue of multiple vs single fans is clear with the Nexus 120. Two of them at 6V move as much air as a single one at 12V, and make 2-3 dBA LESS NOISE!!

I think I missed a step somewhere, with this statement that the CFM moved by two Nexi at 6V equals the CFM moved by a single Nexus at 12V.

Am I incorrectly inferring that there is some kind of universal rule that applies to all fans (CFM vs. RPM vs. voltage (vs. diameter))? Or is this strictly a statement of your own empirical observations of your Nexus 120mm fans that should not be extrapolated to other size/voltage/RPM fans?

The general rule of thumb is that airflow is fairly linearly proportionate to voltage, but not to all speeds. IE, if the fan is just barely spinning, often the airflow is much lower than what the voltage might predict.

I should have mentioned that at 12V, the Nexus fan blows 37~40CFM; at 6V, it blows just about half of that. So, yes, 2 Nexus fans at 6V blows the same CFM as one Nexus fan at 12V.

Pardon me for saying so, but isn't this a somewhat misleading statement? Or rather, this is technically accurate, but can be easily misinterpreted in context of silent PCs.

The implication of arguing 1 fan vs 2 fans is generally in the context of rear only vs. rear + front fans. When put in context like, that it's a different ball park. It suggests that two 6V fans in the rear and front will move the same amount of air as one 12v fan in the rear. I don't believe this is at all true.

(Granted, there is an alternative context available. I once had a case where there was space for 2 80mm fans in the front, and in this sense, it would have been wiser to use 2 fans at half speed rather than 1 at full speed. But this is often not the case.)

Really, I'm going off the original topic, but any time one vs. two fans is brought up, the above subject ought to be mentioned as well.

Fair enough. 2 Nexus 120 fans @ 6V in parallel provides the same airflow as one Nexus 120 fan @ 12V. This is different from one fan blowing out and one blowing in, where the total airflow = the same as a single fan.

My post in the original thread was referring to a system in which there was a Nexus 120 exhaust on the back panel and a 120mm PSU also blowing out.

This only really applies with zero back pressure - if you halve volts you roughly halve amps and quarter the fan power. Any hint of back pressure from an enclosure will reduce the flow far more drastically than if using a single fan at full voltage in the same circumstances.

One reason IMHO why some experimenters get their "best case" cooling with an under-volted fan fore & aft - although flow is no better than that of a single fan, the positive pressure behind the inlet is negated by the negative pressure in front of the outlet, and each fan is giving its maximum (free air) flow for that voltage.
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One thing I'd like to see some checks on - there's an equation for fan noise around showing the noise reduction = 50log(RPM1/RPM2), so (if true) that would be a 15dB drop for running a fan at half the 12V RPM.

Results for the noisier fans in a range (like Panaflo H & M), taking the makers figures, seem to tally fairly well, but it falls down when you move to the quieter (under about 25dBA) fans in the same series. Perhaps your meter could give us some numbers when undervolting the popular models?

I was wondering about this myself a few days ago. To me it makes sense that the 3dBA rule wouldn't hold up in such low ranges. But I'm surprised that two low noise fans at 6v are quieter then one at 12v, defiantly interesting.

Would the results be similar with 80mm L1A's? I'm considering adding a 2nd exhaust fan into my Lian-Li to keep the single fan from ramping up.

forgive me for chiming in so late, but i've only just noticed this thread today

MikeC wrote:

NOTE: A basic rule of acoustic addition states that 2 identical sound sources will result in a 3 dB increase in overall noise. Four identical sound sources will result in +6 dB.

2 Nexus 120 fans, powered by a fanless PSU, in "the quiet room", in which the B&K SLM was seeing an ambient reading of 15~16 dBA (at dawn). Very hard to stop the meter from moving -- even a car 2 blocks away can have an impact when it's this quiet.

OK, so that shatters the idea that 2 identical noise sources means a 3 dB increase. Obviously not a universal rule! I learned something new here!

please be more careful with what you say, it IS a universal rule, it can be very easily derived by definition of dB scale (actually you see a 10*log(2) increase which is 'close enough' to 3dB). this reasoning does not shatter the idea of the '3dB' rule because in fact your methodology is flawed. also, all this talk of the Fletcher Munson curves at the end of your post is a bit misleading, because the main problem here is unrelated to the fact you're using dBA scale. it's not the fault of the A-weighted filter and you would see the same discrepancy with plain old dB if you were able to measure this way. the A-weight (and similarly for the B and C-weighted dB scales) is just an offset for 'percieved loudness' to account for the threshold of hearing. albeit it's made more complicated because the threshold of hearing changes with frequency, but since it is only an offset you should still observe sounds adding logarithmically when using dBA.

the additive error creeps in because you are working at volume levels too close to ambient. a more correct way to analyse the situation at such low volumes is as follows:

you can repeat these calculations for the nexus fan at 12V, and it's a borderline case but you see that the actual fan volume alone (~19.5) is still too small to warrant ignoring ambient. the discrepancy then for 2 fans is only 1 dBA off what you measured, probably within the margin of error for these measurements(?)

the reason why you see the 3dB rule 'working' up at the 30dB fan volume level is because ambient is now insignificant compared to the sample (and approximating the measured fan volume as the fan volume alone is valid).

the fact that you have some PC so close to your ambient, MikeC, makes me wonder whether you'll start to look into trying to reduce the ambient volume now? in absolute terms, your machine is probably now so quiet that you may start getting better results by attacking the room itself. then again, even if it was an easier task, making the room quieter would make your PC appear to sound louder, which is a very backwards kind of 'progress'.

Yeah.....what we really need here is an anechoic chamber. That's the only way to get true measurements of these things. Vanderbilt has one down the street but they never let anyone use it. Maybe if we get a bunch more people to donate Mike can build one himself

This is just personal preference, but I tend to agree with wim; I would prefer if we measured noise in the actual dB scale, not altered by any Fletcher-Munson curves (I swear those things have been following me around since high school). And then have a second half of the review devoted to "subjective" measurements. Even dBA isn't really empirical data, b/c Fletcher-Munson was derived from an imperfect methodology. Just like we've discovered that "average" body temperature is 97.2 degrees and not 98.6, and just like the MP3 codec has been replaced with MP4 (removing slightly different frequency bands as our knowledge grows), I think the curves could be improved upon. It would be nice to see the actual dB level and then have the comments about a low-frequency hum or a 5k clicking or something like that.

Not to say that the reviews are terrible or anything Just a preference.

Hi all, I think I understand what wim is getting at, he may be right. However I like the one that says "when theory and reality conflict, reality always wins." My practical experience was that when I modded my case to take 2 80mm fans at the back instead of the original 1 I was able to get better cooling / noise ratio. I think this is exactly the sort of situation that Mike was trying to measure for.

I'm not living in an anechoic chamber. So I much prefer a method that might not be as scientifically acurate, but shows how humans perceive it in real life situations.

If you read the original discussion, then you'll see that MikeC's original post was meant to counter the "easy answer" you see often when people consider adding a fan. For the practical level we work at the 3 dBA rule is not usable. No matter how many science and math you throw at a single isolated point in MikeC's post, his preliminary and secondary conclusions are what counts. They hold more practical value than any scientific truth.

Our brains are far more sophisticated than any SPL meter or exact calculation. They use fuzzy logic(TM).

Tibors, I'm not trying to refute any point that MikeC made. On the contrary, I agree with him. The 3 dB law is true but what we do here involves too many variables for it to ever be accurate, and we don't always hear what our measurements suggest we should. But I come from the music studio side of things, where we want measurements as empirical as possible, where we use low-pressure HVACs and separate power transformers to remove as many variables as we can. It's just two different ways of looking at the same problem. An engineer can try to quantify data in a problem as much as possible, but at the end of the day it all comes down to what sounds best to the producer's ear.

A very interesting post, but I have a more general question for MikeC - what type of sound level meter are you using that allows you to measure down to 15dBA? I don't know of any sound level meters that will accurately measure down to that level. It appears to me that in this instance you may have simply been "measuring" the inherent noise of your slm.

I don't mean to come over all agressive on my first(!) post but energy doubling 'should' always give you +3dB - if not, then something else must be going on. The fact that you couldn't measure +3dB with 2 identical and incoherent noise sources indicates that 1) they weren't acoustically identical, and/or 2) they were quieter than the ambient level of the room, and/or 3) they were quieter than the inherent noise of your meter.

He used a B&K 2203 Sound Level Meter. It's a lab quality SLM, and I've only seen it once or twice on Ebay. I can't even find a modern day equivalent version. I've heard that it's upwards of $2000+ US to get a modern day equivalent.

Wim has shown that the Energy doubling rule still does apply, even though it doesn't seem apparent. Be sure to read all of this thread. It makes sense. Trust me.

sthayashi, thanks for the info about the meter. I didn't spot that lab tour page - the B&K 2203 is a classic old design (early 70s I recall) and was normally shipped with a Type 4145 mic. I presume that this is the mic that is being used with the meter at the moment?

Anyhow, the thermal noise from the 4145 is quite low at around 10dBA (1 inch mics are usually quite sensitive). But you've also got the electrical noise from the meter itself - on a modern Type 1 meter, electrical noise is usually around 12-15dBA. I've no idea what the overall noise floor of the 2203 is but I'd guess it's at least in the range 10-15dBA. Therefore any measurements made around 15dBA are bound to be susceptible to the effects of internal noise from the meter.

Not long ago I decided to calculate what the increase in dB would be if I had one sound source, and then added a second sound source that produced equal amount of noise. I concluded that the 3dB rule is sort of an estimate, and that to really find the increase in dB one must solve a big equation.

As for the Fletcher-Munson curves, I believe that the actual perceived loudness depends upon the location of the source. It is a function of the azimuth, zenith, and radius from which the source radiates relative to the listener. This function will have slight variations depending on who exactly the listener is. There may be some non-linearities as well, but I don't really know/recall at this time.

The issue of multiple vs single fans is clear with the Nexus 120. Two of them at 6V move as much air as a single one at 12V, and make 2-3 dBA LESS NOISE!!

I don't think there's any way in hell this is true. CFM does not follow voltage, it follows WATTAGE. Anyone here familiar with Ohm's law knows that P = V^2/R.

I don't know what the impedance of these nexus fans are but it doesn't really matter. For this or any fan, running it at 6v will only consume (6^2/12^2) 1/4 of the power and probably run at 1/4th of the speed.

So what do we have here? 1 fan running at 100% speed vs 2 fans running at 25% speed. 3dB is the difference between doubling (or halving) sound output. If we can assume that RPM and sound output are related, then your "discovery" is far from ground breaking.

Let's say this Nexus fan runs at 1600RPM @ 12v. At 6v, this same fan would run closer to 400RPM NOT 800RPM like many would think. This is because of ohm's law. When you reduce voltage by 50%, you reduce power, noise and airflow by 75%. You've basically proven that two fans, together producing half the airflow are half as loud as one fan at full speed.

The 3dB reduction in sound you've discovered is right in line with the 50% airflow penalty.

The issue of multiple vs single fans is clear with the Nexus 120. Two of them at 6V move as much air as a single one at 12V, and make 2-3 dBA LESS NOISE!!

I don't think there's any way in hell this is true. CFM does not follow voltage, it follows WATTAGE.......... yakyakyakyakyakyakyakyakyak............. bawkbawkbawkbawkbawkbawkbawk

The 3dB reduction in sound you've discovered is right in line with the 50% airflow penalty.

You are wrong. And you can't read. From my post above: "A Nexus 120 @ 12V blows 37~40cfm; at 6V, it blows around 18~20 cfm. "

The SPCR lab is equipped with an anenometer, all kinds of rpm sensing devices, multimeters and calibrated power supplies. Direct hands on experimentation and measurements with dozens of fans has shown us that fan RPM is directly proportionate to voltage. And airflow. It is not a perfectly linear relationship, as most fans appear to have some best efficiency speed/voltage/airflow point. But the relationship holds true within a broad linear range of each fan ie, from a bit above the min start / stall speed to just under maximum rpm / voltage overload. Typically, for a 12V fan, this is roughly 6V~ 14V.

Are those numbers from a free-air environment or have you tested them in a case? I'm sure you know actual CFM's drop once you actually install them. Have you taken into account the drop in static pressure? You might be maintaining the free-air theoretical CFM's but CFM's in an actual case will drop because you've pretty much cut the static pressure in half. If this test wasn't done in a case, maybe that's what you should try.

Also, just the fact that you had 16dB of noise floating around when you performed these tests makes them flawed.

Let's say each Nexus fan puts out 13dB

One Nexus fan in absolute silence:

13dB

Measuring the 13dB (@1/2M) fan and the 16dB ambient noise gives:

10*log(10^1.6 + 10^1.3) = 17.76 dB (around what you measured)

The two 13dB fans in absolute silence:

10*log(10^1.3 + 10^1.3) = 16.01 dB

Two fans with 16dB of ambient noise:

10*log(10^1.3 + 10^1.3 + 10^1.6) = 19.55 dB

So in a perfect testing environment one fan (13dB) would be about 3dB quieter than two (16.01dB). Since you had 16dB of noise floating around when you did this test, your results are flawed and it would seem that two fans (19.55dB) are only a little louder than one (17.76dB) for a 1.79dB difference (not unlike what you got). Your test doesn't really shatter much of anything, it was heavily tainted. The ambient noise added up to a third fan. By the way, if you want to be a baby and ridicule me, that's fine, I'm just doing this to share what I know with everyone else.

EDIT: It looks like wim beat me to it. Maybe I should have read the thread before I posted. It would have saved me a lot of time. Anyway you heard it from two different sources, the 3dB rule holds true. Maybe you should fix the post.

PS: Maybe the Nexus fans' electronics straighten out the curve somewhat, or it's just increasing air resistance with speed. I've got an 8K rpm screamer fan that cruises at 2K rpm at 5 volts. With plain motors, light bulbs, heating elements, loudspeakers or any other ohmic device, ohms law applies. Just like with any device that emits sound, the dB scale applies.

Of course the cfms would drop with greater impedance. No disputing that. As to how much at different rpm/voltage, my guess is proportionately, but I would not claim to know till a few real fans were tested in controlled conditions.

Of course noise perception / measurements would change if you had lower noise. The question would be Who has 13 dBA background noise? Who has absolute silence? (You live in outer space or something?)

I don't go around sticking foot in mouth saying "no way in hell" about anyone else's results without clear empirical knowledge.

You seem to have missed the whole point of the original post. It's certainly no challenge to the "laws" of acoustics, and my use of the word shatter was to used only to describe my initial surprise, which later in the post I explained was wrong.

The real point is simply this:

2 parallel fans spinning at half the speed will provide the same airflow as one spinning at full speed, and in a quiet room (which 16 dBA certainly is) will be perceived as quieter than the one fan because the slow spin speed drops the frequency of the noise to a range where we simply cannot perceive it as well.

It is a practical piece of information that can help in designing a PC that is subjectively, perceptively quieter.

This information would certainly be more useful if it was tested with fans in a real case. But the choice of fans, the way they are mounted in the case and the choice of caseitself would also impact the results.

1) the fans would have to be effectively decoupled from the case to prevent any vibrations from causing additional noise.
2) If the 2 fans at half speed make more vibrations than one at full speed, and the fans are direct-screw mounted, it's possible for the vibration-induced noise from the case panels to swamp out direct fan noise differences.
3) If the case is particularly flimsy & the fans have high vibration, then, the fan vibrations could be more important than the direct acoustic fan noise.

Finally, the reality is that I have built many multiple fan systems with perceived noise levels at or below the threshold of background noise. The entire first post is a counterpoint to the instinct that tells most people that fewer fans will be better (read: quieter) than more fans. My point is that it ain't necessarily so.

Of course the cfms would drop with greater impedance. No disputing that. As to how much at different rpm/voltage, my guess is proportionately, but I would not claim to know till a few real fans were tested in controlled conditions.

I'm glad we can agree on that.

What I was trying to say was that your 1 fan 12v setup will not suffer as great a CFM drop as your 2 fan 6v setup once installed because of static pressure. Not to mention that you'll need huge amounts of open exhaust area on the case (if the 2 fans are sucking in). I know that using a fan X on a case Y and posting the results wouldn't be helpful to every single person here but it would give a real world performance example. Since you have equipment that can measure CFM, you should do a spec sheet vs real world performance test. Saying my car can do 0-60 in 1.3 seconds while on jackstands isn't likely to impress anyone, I think the same applies here.

PS: If your goal was to benchmark perceived sound output, you shouldn't have used a dB meter. Maybe a blind A/B test involving your staff would have been more appropriate.

It's easy enough to hear without resorting to such efforts. The difference was plainly audible -- in my test conditions with the fans I used.

So if two other people listen and they can't hear what I hear, what does that prove? That my hearing is better than theirs? And lots of others' hearing will be better than mine...

Just try it for yourself. You don't have to trust me or my perceptions, the tools are out there for anyone to buy and use.

BTW, I'd never use two intake fans and I'd never have any fan in the front of the case running anywhere higher than ~800rpm. High back pressure from high impedance is simply not an issue with any PC I build. I'd also never have an 8k screamer running at any speed except off.

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